Several of the major applications for tantalum are in electrolytic capacitors and in furnace components in high temperature, vacuum furnaces. The properties of tantalum that makes it an attractive material for use in these applications include: high melting point, high dielectric constant in the tantalum oxide film formed by anodizing; high electrical conductivity; excellent ductility and fabricability; and availability in high purity forms.
It is known that tantalum is embrittled when exposed at relatively high temperatures (above 600.degree. F.) for even short periods of time to certain gases and vapors. Oxygen, carbon monoxide, and carbon dioxide are notable examples of contaminating gases, and they are important since they comprise the products or reactants of many physical and chemical reactions involving the use of tantalum products, either directly or indirectly, in the electronics, metal and chemical industries. The above-described embrittlement condition refers to the loss of suitable ability to bend without breaking in the intended application (as observed and measured at or near room temperature) resulting from exposure of tantalum to high temperatures in unsuitable vacuums or in contaminating gases and vapors. The lack of low temperature ability to bend without breaking after contamination causes severe problems since fabricated parts of tantalum that have been contaminated are subsequently exposed to vibration, impact and static forces at or near room temperature during their life or during their manufacture into certain finished products or devices.
One of the major difficulties in the use of tantalum in electrolytic capacitors has been that tantalum lead wires often become severely embrittled during sintering of slug-type tantalum anodes produced by pressing and sintering of tantalum powder with the tantalum lead wire embedded in the powder slugs. The extent of embrittlement is known to be more severe when such tantalum lead wires are embedded in tantalum powders having a relatively high oxygen content, for example, more than 1600 parts per million.
One method that has been used as a means to attempt to overcome this difficulty has been to treat the surface of the tantalum lead wire with carbon or a carbonaceous material. The carbon coating tends to react with oxygen in the tantalum powder during the subsequent high-temperature sintering operation, and thus by this means the bendability of the lead wire is maintained because the oxygen has reacted with the carbon coating rather than being absorbed by the tantalum lead wire. With this method, however, it is difficult to control the application of carbon to obtain consistent behavior and maintain the necessary bendability in the lead wire. In addition, carbon on the surface of the wire exerts an adverse effect on the electrical properties of the tantalum by producing an undesired increase in DC leakage through the tantalum oxide dielectric film.
Still another method that has been used in an effort to lessen the extent of embrittlement of the tantalum lead wire is to use a grain-size-controlled tantalum lead wire; i.e., a tantalum wire that exhibits a grain size that does not grow significantly upon exposure to the elevated temperatures employed during sintering of the anodes. However, the grain size control lead wire does still not have the desired resistance to embrittlement in many instances, especially in those applications where the grain-size-controlled tantalum lead wire is embedded in a high-oxygen containing tantalum powder slug and especially where the oxygen content of the tantalum powder is 1600 ppm or higher.
The embrittlement of tantalum lead wires is a principal and major problem when handling anodes when they are welded to the anodizing rack. The embrittlement is most severe in the tantalum lead wire at a location adjacent to the high oxygen-containing powder where the anode lead wire or riser is embedded in the tantalum powder slugs. The principal embrittlement is noted at the point of egress of the wire from the sintered anode, where the oxygen content in the wire is high and where the wire is unsupported. The embrittlement of tantalum lead wires has a strong bearing on whether or not a capacitor manufacturing operation can be operated economically or not, because failure of the lead wires during handling can lead to a complete loss of the parts. Thus, all value added to the material through that stage of manufacturing is lost. A solution to the tantalum lead wire embrittlement problem has a strong bearing on making capacitor operations economical. Normally the embrittlement problem is most severe in tantalum capacitor anodes that are pressed and sintered from powders containing relatively high oxygen content (e.g., 1600 ppm oxygen or higher), and with powders sintered at temperatures of 1800.degree. C. or higher.
Furthermore, embrittlement of wrought tantalum fabricated components in high temperature furnace or other high temperature applications can adversely affect life of the parts. Tantalum materials in high temperature applications are adversely affected because they are getters for gases such as carbon monoxide or dioxide, oxygen and nitrogen. Because of the high cost of fabricated tantalum parts, replacement of the parts because of embrittlement can cause lengthy down-time and result in a sizable replacement cost. Substantial economical benefits can be gained if the service life of such tantalum parts can be increased.